The teen may have been dancing on a seat cushion with his back to the freeway signs when his head and hands slammed into the sign

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Mason Zisette, a 16-year-old Manhattan Beach boy, died Saturday, July 12, 2014, days after he struck a freeway overpass when he stood up on an open roof double-decker bus traveling 45 mph, police said.

The double-decker bus with Starline Tours chartered for a private party was traveling south before 8 p.m. Thursday on the San Diego (405) Freeway near the Arbor Vitae Street overpass when the incident happened, said CHP Officer Edgar Figueroa.

The teen was identified as Mason Zisette, according to the coroner's office.

The private bus had 45 passengers, most of whom were under 18 years old and were with adult chaperons.

A witness told NBC4 that the teen may have been dancing on a seat cushion with his back to the freeway signs when his head and hands slammed into the sign. He then collapsed on the floor, the witness said.

About two dozen teens were present and dancing on the upper deck at the time of the incident. All adult chaperones were inside the bus at the time.

The California Highway Patrol was investigating the incident. Starline declined to comment.